While for some people this may be a killer feature, I hardly use Facebook or Twitter and have no plans to share content through those services. Email suits me just fine. As such, I didn't want to have those listed in Mountain Lion's Share Sheets.
So, I grepped for Facebook and got this:
$ find /System -name Facebook -print 2>/dev/null
/System/Library/InternetAccounts/Facebook.iaplugin/Contents/MacOS/Facebook
/System/Library/PrivateFrameworks/ShareKit.framework/Versions/A/PlugIns/Facebook.sharingservice/
Contents/MacOS/Facebook
/System/Library/SocialServices/Facebook.socialplugin/Contents/MacOS/Facebook
And for Twitter:
$ find /System -name Twitter -print 2>/dev/null
/System/Library/PrivateFrameworks/ShareKit.framework/Versions/A/PlugIns/Twitter.sharingservice/
Contents/MacOS/Twitter
Preferring to do the least damage possible, I simply renamed Facebook.sharingservice to Facebook.sharingservice.old and Twitter.sharingservice to Twitter.sharingservice.old. After doing this, I quit Safari, reopened it, and gone were the Facebook and Twitter options.
No guarantees this won't reenable on updates or break something else, but so far it seems to work. If nothing else, you can always rename the plugins back.